This invention relates to a power transmission control apparatus. More particularly, the invention relates to a power transmission control apparatus comprising a main change speed control valve of a power transmission, a hydraulic pump for supplying pressure oil to the main change speed control valve, hydraulic cylinders for controlling clutches, a modulation valve disposed between the main change speed control valve and the hydraulic pump for gradually increasing a hydraulic pressure for the hydraulic cylinders after the hydraulic cylinders are filled with the pressure oil by operating the main change speed control valve to connect the hydraulic pump to the hydraulic cylinders, and a backward and forward switch valve connected to an output port of the modulation valve through an inching valve for controlling the clutches. Such an apparatus has a modulating function played by the modulation valve whereby a vehicle installed with this apparatus does not make a sudden start when the main change speed control valve is operated, and a clutching function in which a depression of the clutch pedal causes the inching valve to stop oil supply to the backward and forward switch valve thereby stopping the vehicle running.
Conventionally this type of control apparatus is devised to permit the vehicle to make a slow or gentle start only, i.e. to start at a speed corresponding to an amount of return of the clutch pedal after the clutch is disengaged, when the clutch pedal is suddenly released after the clutch is disengaged, a great amount of oil is supplied from the output port of the modulation valve to the backward and forward switch valve through the inching valve. This lowers a hydraulic pressure at the output port of the modulation valve, resulting in the modulating function. Since it takes time to recover a required pressure, the vehicle is incapable of a sudden start.
Consequently, the conventional control apparatus has the disadvantage that the working vehicle cannot carry out an efficient operation, for example, by means of a sudden start to cause a shovel loader to burst into soil.